The invention relates to a method for detecting diffusely scattering materials, impurities, deposits, damage or coatings of the surface or fluctuations in the material thickness in items made of transparent material, the items being transilluminated and examined by a light source and a camera.
Structureless, semi-transparent impurities of items made of transparent material such as glass, PET, PC, PVC and similar can often not be detected using conventional image-processing methods. Among such impurities are for example milky adhesive tapes, thin lacquer coats, rust and thin partly mineral deposits on the inside or outside of the wall of the items. Furthermore, the detection of changes on the surface of the transparent material, such as scratches covering large areas, abrasion traces, surface parts which are etched or sand-blasted as well as in general coatings which impair transparency is also problematic. Such semi-transparent defects slightly attenuate the light entering the camera from the light source in a straight line and diffuse it only slightly. Observed over a larger area, such a defect scarcely reduces brightness, particularly as a dispersion of the light which is brought about by the transparent material itself must also be taken into account. In processes which work with bright-field illumination, the detection of such defects or irregularities is therefore scarcely possible, as the image recorded by the camera shows neither contrasts, contrasts which could be additionally intensified by image processing methods, nor a great reduction in the brightness of the image.
Dark-field methods which are based on a change in the polarization of the light by defect to be detected (EP-A-0 387 930) are often not usable due to the polarization effects which occur in transparent container materials themselves. Other dark-field methods in which the optical axis of the camera stands at a right angle to the direction of illumination (EP-A-0 429 086) can often be carried out only with difficulty due to geometric boundary conditions, and the scatter caused by the semi-transparent defects is often not large enough for these processes.